Devices for controlling the spatial distribution of the intensity of electromagnetic waves, such as light, are often designated as spatial light modulators (SLM's). Such devices which can be used in processing data are capable of spatially modulating a collimated coherent or incoherent beam of light with, for example, input data which is to be processed. The devices are appropriately coupled to optical data processing systems into which the data modulated light beam is supplied at a rate commensurate with the processing system's potential throughput, the optical processing system utilizing parallel processig without the limitations normally imposed by serial manipulation of the data.
Many different forms of spatial light modulators have been suggested by those in the art. An early article, entitled "Spatial Light Modulators", by David Casasent and published in the Proceedings of the IEEE, Vol. 65, No. 1, January 1977, at pages 143-157, provides a summary of various types of spatial light modulators that have been suggested by the art. The devices described therein include SLM's using liquid crystal materials; materials which undergo surface deformation patterns (sometimes referred to as deformable SLM's), i.e., thermoplastic materials, dielectric oils, ruticon, or elastomers, or membranes combined with surface channel charged coupled devices (CCD's); alkali halide materials having intentionally introduced color center defects (sometimes referred to as photodichroic SLM's); materials which exhibit the Pockels effect (sometimes referred to as Pockels SLM's); materials using ferroelectric-photoconductor characteristics; materials using ferroelectric-photorefractive characteristics; and SLM's using acousto-optic techniques, magneto-optic techniques, techniques utilizing the characteristics of amorphous semiconductor materials; and techniques using magnetic-bubble devices. A later less detailed, up-dated summary was more recently published. Such article, entitled "A Review of Spatial Light Modulation", by A.D. Fisher, presented at the Topical Meeting on Optical Computing, sponsored by the Optical Society of America, at Incline Village, Nevada, Mar. 18-20, 1985, briefly discusses various devices and the general status of the art at that time.
In most cases, however, the devices discussed in the art are only optically addressable by using a scanning light beam, for example, or electron beam addressable by using a scanning electron beam. Such devices are cumbersome and expensive to fabricate and are slow in operation. Of the relatively few types of devices which are electrically addressable, such as devices which use membranes deflected by electrical signals which are supplied through electrodes in contact with the membrane or devices which use membranes combined with charged coupled devices, the structure thereof is extremely difficult to fabricate and the membrane response is relatively slow so that such devices are not readily usable for high speed, real-time processing operations.
A spatial light modulator which operates in real time and which is primarily, and often preferably, electrically addressable or, alternatively, is optically addressable, has been disclosed in U.S. patent application Ser. No. 224,140, filed on Jan. 12, 1981 by R. H. Kingston and F. Leonberger, now issued as U.S. Pat. No. 4,696,533 on Sept. 27, 1987. The spatial light modulator disclosed therein is a relatively compact device handling a relatively large amount of input data in a relatively small volume, the device being capable of operation at high speeds, using up to as high as 10.sup.9 data samples per second. The device utilizes a suitable substrate having a "buried channel" charge-coupled device (CCD) formed at a surface of the substrate. The amount of charge in the charge storage wells associated with a plurality of electrodes of the buried channel CCD is controlled by an electrically or optically addressed data signal. In the specific embodiment disclosed therein, the level of charge in such charge storage wells thereby controls the electric field beneath the electrodes such that the transmitted light is spatially modulated by the charge levels in the charge storage wells in accordance with the Franz-Keldysh electro-absorption effect.
A problem with such a device is that the amount of modulation of an electromagnetic wave that can be achieved is very limited when using the Franz-Keldysh effect, e.g., modulation levels of less than 20%-30% can be obtained. It is desirable that much greater electro-absorption effects be achieved, while still maintaining the ability to do so for a two-dimensional (2D) wave using a 2D array of such devices.